Oak wilt is a disease that is killing oak trees in southwestern Pennsylvania. It is particularly bad in the North Hills suburbs. Sandy Feather, agricultural agent for the Allegheny County Cooperative Extension, detailed the disease so that homeowners could be alerted to the problem. This article is a summary of her findings:

All oak trees can be infected, but the red oaks are the most susceptible.

Red oak tree leaves have pointed lobes and include northern red oak, pin oak, black oak and scarlet oak.

Oak wilt symptoms on red oak generally start at the top of the tree and progress downward. Leaves turn dull green, then bronze or tan. Browning is frequently evident at the leaf tops or margins. Sometimes the leaves droop and curl lengthwise. Leaves at the branch ends begin to fall soon after symptoms become noticeable. Twigs and branches die, and discoloration continues for several weeks until the tree is dead.

Symptoms for white oak are similar, but advance much more slowly and do not cause sudden defoliation and death. The symptoms are often confined to a single branch and can look like typical fall coloration. When white oak is infected, it might decline over a 2 or 3 year period or even go into remission.

Oak wilt is caused by a fungus. The disease kills infected trees by clogging their vascular systems until they are unable to transport water and nutrients through the tree. The fungus is transmitted from infected trees to healthy ones by oak barn beetles and sap beetles.

These beetles are particularly attracted to fresh pruning wounds. To minimize the possibility of infection, prune trees between November and mid April. Oaks are most susceptible in spring and early summer as new growth occurs.

Another way oak wilt is spread is through naturally occurring root grafts. Oak trees growing near one another often have their roots growing together. The disease is easily transmitted from the infected tree to the healthy one through shared roots. To protect the uninfected tree, the root grafts must be broken. The can be done by digging a trench between infected and healthy trees, or by killing the grafts chemically.

Infected trees should be removed as soon as possible to protect healthy ones. There is no chemical control once symptoms are apparent in more than 30 percent of the crown. Destroy the wood immediately, including the stump, by burning or debarking so that it is not attractive to beetles. Do not stack the firewood or transport logs with intact bark since insects in the infected wood can leave and carry the fungus to healthy trees.

To determine if your oak trees are infected with oak wilt, contact a certified arborist. The arborist can diagnose if the trees are diseased and can safely remove dead trees. If less than 30 percent of the crown is affected, injection of a fungicide can put the disease into temporary remission. Root grafts between the infected trees and neighboring oaks should be broken because the fungicide does not kill the fungus in the tree roots.

An arborist can also advise you about protecting uninfected oaks with an injection of the fungicide every other year. Several certified arborists can be found online at www.isaarbor.com.

You also may have heard about sudden oak death, a new disease found in California and Oregon. This disease has not yet been found in Pennsylvania. This disease has similar symptoms to oak wilt and has killed thousands of oaks in the west. The U.S. Department of Agriculture has imposed strict regulations on shipments of nursery stock from the area.